Editorial: The role of nitrate in plant response to biotic and abiotic stress

COPYRIGHT © 2023 Limami and Hirel. This is an openaccess article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. TYPE Editorial PUBLISHED 06 July 2023 DOI 10.3389/fpls.2023.1240256

mediation. In this Research Topic several new nitrate transporters are described and their roles as transporters and sensors are characterized. Wu et al. functionally characterized NtNRT1.1B in a heterologous system namely the nitrate-uptake defective yeast HpDynt1, in which the plant NtNRT1.1B enabled growth recovery. Cellular and tissue localization showed that NtNRT1.1B is targeted to the plasma membrane and that the cognate gene is expressed in the root stele and shoot-stem vascular tissues. Such findings suggest that NtNRT1.1B plays a role in long distance transport. Transgenic tobacco overexpressing NtNRT1.1B exhibited higher biomass, which allowed the authors to propose that this gene can be a target for improving N-use efficiency.
By using bioinformatics and RNA-seq Cheng et al. identified 92 and 88 putative NPF genes from Setaria italica L. and its wild ancestor Setaria viridis L. respectively. SiNPF gene family exhibited high number of duplication events. The authors identified 26 tandem duplications and 13 segmental duplications. The response of SiNPF gene family to low nitrate stress was analyzed by a series transcriptomic analysis performed during plant growth. This approach suggested that the tandem SiNRT1.1B.1 and SiNRT1.1B.2 from Setaria italica L. (Foxtail millet) could contribute to low N tolerance in this species. Functional characterization of both genes in Yeast and Arabidopsis by complementation of mutants confirmed their nitrate transport activity.
In the legume Lotus japonicus Rogato et al. characterized and described the role of a high affinity nitrate transporter, LjNRT2.1, in the response to changes of N availability in the surrounding environment. Characterization of two Ljnrt2.3 knock out mutants showed that LjNRT2.3 fulfils a nutritional and signaling role as it is involved in root nitrate acquisition and signaling controlling lateral root elongation in response to N starvation. This characterization also suggested an epistatic interaction between LjNRT2.3 and LjNRT2.1 as the induction of LjNRT2.1 when nitrate availability is low was abolished in Ljnrt2.3 mutant.
Vacuoles are important storage compartments for nitrate. They allow the cell to cope with an excess nitrate when its uptake exceeds N utilization and mobilization when plants N demand is not met by N availability. Lu et al. work is dedicated to the tonoplast-localized nitrate transporters, namely NPF5.10, NPF5.14 and NPF8.5. Expression of the corresponding genes in Xenopus laevis oocytes confirmed their nitrate transport activity. Overexpression of these genes in Arabidopsis reduced nitrate content in the vacuoles supporting their role in vacuolar nitrate efflux. Interestingly the six genes belonging to the NPF family (NPF5.10, NPF5.14, NPF8.5, NPF5.11, NPF5.12, and NPF5.16) were all induced by mannitol. Furthermore, nitrate accumulated more in the sextuple mutant than that in the wild type after mannitol treatment. Altogether these findings allowed authors to propose that nitrate efflux from the vacuole to the cytosol could contribute to plants response to osmotic stress rather than to mineral N nutrition.
In order to get better insight on the complex mechanism of genes regulation by nitrate in apple (Malus domestica) Wen et al. identified the function of the transcription factor (TF) MdHHO3 ((HYPERSENSITIVITY TO LOW PHOSPHATE-ELICITED PRIMARY ROOT SHORTENING1 HOMOLOG 3) a member of the GARP family. These authors showed that MdHHO3 inhibits genes transcription by binding to a motif-containing GAATC present in their promotor. Accordingly, authors showed that MdHHO3 negatively regulated the expression of the nitrate transporter MdNRT2.1 by binding to its promotor. Characterization of Arabidopsis and tobacco overexpressing MdHHO3 showed that the transcription factor controls a large portfolio of genes related to nitrate nutrition. In MdHHO3 overexpressors, nitrate transport-related genes were downregulated and nitrate assimilation-related genes were upregulated. Furthermore, senescence was induced in Arabidopsis and tobacco overexpressing MdHHO3 under nitrate deficiency.

Author contributions
AL wrote the first draft that was reviewed and finalized by AL and BH. All authors contributed to the article and approved the submitted version.

Funding
Our research is supported by university of Angers and INRAE (Institut national de recherche pour l'agriculture, l'alimentation et l'environnement).